1. Field of the Invention
The present invention relates to telephone subscriber supervisory circuits and more particularly to a circuit for connecting and disconnecting a ringing voltage generator to a telephone subscriber loop without allowing harmful transient discharges to develop.
2. Description of the Prior Art
When a telephone subscriber's loop circuit is switched between a DC transmission source and an AC ringing voltage generator, substantial transients are typically generated by the interruption of these circuits, which are inductive in nature. These transients are particularly harmful in digital network telephone offices.
This problem has traditionally been solved by resistor/capacitor networks or other types of contact suppression circuits. A contemporary approach has been to coordinate sophisticated electronic instantaneous line state sensing or preconditioning schemes with the transfer of the ringing voltage generator to and from the subscriber's loop, as disclosed in U.S. Pat. No. 4,071,709 to D. Q. Lee, et al. However, these methods all incur substantial penalty in the form of cost, size, and/or limitations on system performance.
Accordingly, it is an object of the present invention to provide a low cost, maintenance free technique of switching the telephone subscriber's loop between a DC transmission source and an AC ringing voltage generator without generating harmful electrical transients.